Formability enhancement for tailor -welded blanks.

Abstract

The revolutionary concept of tailor-welded blanks (TWB) was introduced into the automotive industry in the earlier 1980's. The number of TWB panels has grown significantly in the recent years because of their great benefits. The most important benefit of TWB is that it distributes the weight or the special material to the place exactly it is needed. However, technical challenge exists in forming TWB: in general, forming non-homogeneous blanks narrows the formability window, and it causes undesired weld line movement. To date both fundamental understanding and practical solution to the formability and weld line movement control are still ambiguous. In this thesis, a simplified model has been developed to analyze the deformation of TWB under uniaxial tensile loading and with orientated weld line. In addition to material properties, the strain/stress distribution in TWB has been found also related to the geometrical configuration of the specimen, for example, the weld line orientation and base metal length ratio. Such configuration dependence of TWB has been summarized and general guidelines for designing TWB have been obtained. This thesis then proposes two methodologies to control the weld line movement in order to improve the TWB formability. One is using blank holding force (BHF) control and the other is using drawbead design. A simplified 2-D analytical model has been developed to estimate the stress/strain distribution and the required BHF or drawbead restraining force (DBRF) for different base metal sides. The model can be further extended to 3-D cases by superimposing the 2-D section analysis results around the entire binder ring. The required BHF or DBRF level is related to the geometry profile and determined by the friction force and bending force developed for each base metal side. Finite element simulations and experiments have been conducted to verify the proposed control methodologies. As a result, the proposed BHF control strategy and drawbead design are proved to be effective in controlling the weld line movement and enhancing the TWB formability.